The most common commercial form of injection molding machine is known as a xe2x80x9creciprocating screwxe2x80x9d. In this type of machine, thermoplastic polymer is melted, mixed, and conveyed by means of a screw having one or more flights rotating within a heated pressure vessel. The screw is also permitted to translate axially to allow for the accumulation of melted material at the end of the screw. When sufficient melt has accumulated, the screw is stopped and translated forward to inject the melted material into a closed mold. In common practice a non-return valve is situated at the downstream end of the screw to prevent back flow into the screw flights during the injection portion of the cycle.
A variation of this process, known as co-injection or sandwich molding, has been commercially practiced for a number of years. In its most common embodiments shown in FIGS. 1 and 2, co-injection is achieved by means of a molding machine fitted with two or more plasticizing units, each one containing a reciprocating screw 12, 14 enclosed in a separate heated barrel 16, 18. The output of these plasticizing units is brought together by a system of manifolds 20 which convey the several materials to the point of injection 22 into the mold 24. According to well known principles of viscous flow, the first material to enter the mold remains substantially on the outside of the molded part, and material injected later remains substantially in the core of the part. The resulting xe2x80x9csandwichxe2x80x9d construction yields a number of advantages, the principle ones being: (1) to make a part with a chemically foamed core, gaining the light weight, low pressure, and flat surfaces of a foam part without the characteristic streaky exterior, (2) to use low cost recycled, xe2x80x9coff-specxe2x80x9d or uncolored material where it is not visible, and (3) to make a part with different properties on the inside and outside, as for instance the presence or absence of reinforcing fibers or other property-changing additives.
These advantages are offset by the high cost and complexity of a machine requiring two or more independent reciprocating screws, together with the associated controls for simultaneous and/or sequential injection. Numerous attempts have therefore been made to reduce this complexity by having at least the injection function be performed by a single element, building a composite shot containing a plurality of melted materials within a single accumulation space. Examples of these attempts include U.S. Pat. No. 4,978,493 to Kersemakers et al., U.S. Pat. No. 3,966,372 to Yasuike et al., and U.S. Pat. No. 5,443,378 to Jaroschek et al. In all of these examples, at least one secondary extrusion screw and barrel is caused to communicate with the primary barrel by means of a melt-carrying manifold structure through which the secondary portion of the shot is charged. Because of the multiple barrels and screw drives, machines of this type still have disadvantages involving the high initial cost of the required components and associated control capability.
One object of the present invention is to provide a device and method whereby the sandwich molding process can be performed within the space of a single heated barrel, and furthermore by means of a machine having a single rotational screw drive.
Another object of the present invention is provide a device and method for making possible adaptation of a conventional single-material molding machine to perform the sandwich molding process with relatively minor and low-cost changes.
A further object of the present invention is to provide an injection unit having an externally heated pressure vessel hereafter called the xe2x80x9cbarrelxe2x80x9d enclosing a primary, outer, screw having one or more flights. The screw being movable rotationally and axially to respectively plasticize and inject a metered quantity of melted thermoplastic material. The screw has a hollow cylindrical bore extending through its length which forms the enclosure for a second, inner, screw having a smaller diameter than the primary screw. The secondary screw remains substantially fixed in its axial orientation with respect to the primary screw, but can be rotated within the primary screw to plasticize material. The downstream end of the bore in the primary screw is provided with an opening to allow material from the secondary screw to be extruded into an accumulation space in front of the primary screw. The opening is preferably provided with a non-return valve to keep material from leaking back into the secondary screw.
Another object of the present invention is to provide a clutch mechanism between the primary and secondary screws to transmit rotation to the respective screw elements. Rotational motion is applied by means of a hydraulic or electric motor to the upstream end of the secondary screw. With the clutch closed, relative rotation occurs only between the primary screw and the outside barrel causing primary material to be extruded. With the clutch opened, relative rotation occurs between the two screws, causing secondary material to be extruded.
Another object of the invention is to provide a secondary feed throat to convey granular raw material to the inner screw through openings cut through the wall of the hollow outer screw.
A further object of the invention is to provide a machine for forming objects having: (1) a first rotatable screw; (2) a second rotatable screw; and (3) a single screw drive for selectively causing rotation of the first rotatable screw and the second rotatable screw. This machine can also include: (a) a single barrel containing the first rotatable screw and the second rotatable screw; and/or (b) a clutch for selectively connecting the single screw drive to one of: (1) the first rotatable screw; and (2) both the first rotatable screw and the second rotatable screw. Furthermore, the first rotatable screw and the second rotatable screw can be substantially coaxial. Also, the first rotatable screw can operate on a first material; and the second rotatable screw can operate on a second material such that each of the objects is formed from a combination of the first material and the second material.
An additional object of the invention is to provide a machine for forming objects having: (1) a barrel; (2) a first rotatable screw; (3) a second rotatable screw; and (4) an accumulation space for forming a shot, such that the barrel surrounds at least a portion of each of the first rotatable screw, the second rotatable screw, and the accumulation space. Also, the first rotatable screw can operate on a first material; the second rotatable screw can operate on a second material; and the shot formed in the accumulation space can contain first material and second material. Further, the first material can be a skin material; the second material can be a core material; and at least one of the first rotatable screw and the second rotatable screw can inject the shot into a mold such that skin material encapsulates core material. In addition, the accumulation space can enlarge when at least one of the first rotatable screw and the second rotatable screw move axially.
Another object of the invention is to provide a screw assembly for a machine for forming multi-material objects having: (1) an outer screw for operating on a first material, the outer screw having a hollow inner section; and (2) an inner screw for operating on a second material positioned substantially within the hollow inner section of the outer screw; wherein at least one of the outer screw and the inner screw includes a non-return valve to prevent backflow of material into at least one of the outer screw and the inner screw. In addition, each of the outer screw and the inner screw can include a respective non-return valve. Further, the non-return valve can be selected from the group consisting of: a ball type valve, a ring type valve, and a poppet type valve.
A further object of the present invention is to provide a machine for forming objects having: (1) a hollow barrel defining a longitudinal direction; (2) a screw for operating on a material in the barrel and having an elongated tip, the screw movable in the longitudinal direction; and (3) a nozzle having an orifice for passage of material from the barrel, and a tip portion for receiving the elongated tip of the screw. In addition, the screw can have a hollow bore which continues into a passageway through the elongated tip to an outlet in the tip. Also, a second screw can be positioned within the hollow bore for operating on a second material in the hollow bore and/or the nozzle can have a storage area for retaining material until the elongated tip substantially reaches an inner face of the orifice.
An additional object of the invention is to provide a method of forming multi-material objects having the steps of: (1) engaging a screw drive to cause a first screw to extrude a first material into an accumulation space; (2) engaging the screw drive to cause a second screw to extrude a second material into the accumulation space forming a composite shot; and (3) injecting the composite shot into a mold by axial movement of at least one of the first screw and the second screw. Also, the step of engaging the screw drive to cause the first screw to extrude can include the step of increasing pressure on a clutch disc; and/or the step of engaging the screw drive to cause the second screw to extrude can include the step of decreasing pressure on the clutch disc.
Another object of the invention is to provide a method of forming multi-material objects having the steps of: (1) rotating a first screw to cause a first material to be extruded; (2) forming by axial movement of at least one of the first screw and a second screw an accumulation space having a forward side, a rearward side, and a periphery, such that the first material accumulates toward the forward side of the accumulation space; (3) rotating the second screw to cause a second material to be extruded into the accumulation space; (4) enlarging the accumulation space by additional axial movement of at least one of the first screw and the second screw, such that the second material accumulates toward the rearward side and outer periphery of the accumulation space; and (5) forcing the first and second material in the accumulation space into a mold. In addition, the axial movement can be caused by the first material exiting the first screw; and/or the additional axial movement can be caused by the second material exiting the second screw. Also, the first material can be a skin material and the second material can be a core material.
A further object of the invention is to provide a method of plugging-off a multi-material object having a core material encapsulated by a skin material having the steps of: (1) filling a storage area in at least one of a barrel and a nozzle with a portion of a skin material, the storage area positioned adjacent a path of an elongated tip of an injector; (2) moving the injector axially such that the elongated tip of the injector is received into a tip portion of the nozzle thereby injecting skin material and core material into a mold; and (3) further moving the injector such that another portion of the injector moves into the storage area thereby causing injection of the portion of skin material into the mold to plug off the object.
An additional object of the invention is to provide a method of plugging-off a multi-material object having a core material encapsulated by a skin material having the steps of: (1) filling a storage area in an elongated tip of an injector with a portion of skin material; (2) moving the injector axially such that the elongated tip of the injector is received into a tip portion of the nozzle thereby injecting skin material and core material into a mold; and (3) extruding the portion of skin material from the storage area into the mold to plug off the object.
Another object of the invention is to provide a machine for forming multi-material objects having a first screw for operating on a first material and a second screw for operating on a second material within the same barrel, each screw having a non-return value preventing backflow into either screw; and an accumulation space formed by the axial retraction of one or both screws.